Myotonia Congenita

Subdivisions of Myotonia Congenita

General Discussion

Myotonia congenita is a rare genetic disorder in which an abnormality of voluntary (skeletal) muscle fiber membranes causes an unusually exaggerated response to stimulation (hyperexcitability). As a result, affected individuals have difficulty relaxing certain muscles after contracting them (myotonia), muscle stiffness (rigidity), and associated symptoms. Such symptoms tend to occur when attempting to move certain muscles after rest. In many cases, individuals with myotonia congenita also have abnormal enlargement of the muscles (hypertrophy), resulting in a "herculean" or "body-builder like" appearance.

Two main forms of myotonia congenita have been described: Thomsen disease and Becker disease. In individuals with Thomsen disease, symptoms and findings such as myotonia, associated muscle rigidity, and abnormal muscle enlargement may become apparent from infancy to approximately two to three years of age. In many cases, muscles of the eyelids, hands, and legs may be most affected. Thomsen disease is transmitted as an autosomal dominant trait.

In those with Becker disease, symptoms most commonly become apparent between the ages of four and 12 years. As in Thomsen type myotonia congenita, affected individuals develop myotonia, associated muscle rigidity, and abnormal muscle enlargement (hypertrophy). The symptoms tend to remain constant, with little progression. Becker disease is inherited as an autosomal recessive trait.

Signs & Symptoms

Myotonia congenita is a disorder of voluntary (skeletal) muscle characterized by an exaggerated response to stimulation of certain muscle fibers (hyperexcitability). As a result, there is painless spasm (tonic spasm) of affected muscle after forceful voluntary contraction, abnormal muscle stiffness, and difficulty relaxing muscle (myotonia). Such symptoms tend to occur when affected individuals attempt to move certain muscles following a period of inactivity or when affected muscles are mechanically stimulated. Characteristic symptoms may include difficulties relaxing hand grip; completely opening the eyes after forcibly closing them (e.g., following crying or sneezing); arising from a chair; starting to walk or run; climbing stairs; and/or arising from bed at night. As noted above, the myotonia is characteristically most pronounced following a period of rest. In addition, it typically improves by gradually increasing the force or intensity of movements through “warming up.” Most individuals with myotonia congenita also develop abnormal enlargement of voluntary muscles (hypertrophy), resulting in an athletic appearance.

As noted above, two major forms of myotonia congenita have been described: Thomsen disease (autosomal dominant type) and Becker disease (autosomal recessive type). (For more information on mode of inheritance, please see the “Causes” section of this report below.)

In Thomsen type myotonia congenita, symptoms may become evident from infancy to approximately two to three years of age and are typically nonprogressive. However, the severity of the myotonia may vary greatly among members of the same family, with males typically more severely affected. According to some reports, exposure to cold temperatures may aggravate symptoms in some cases.

In many individuals with Thomsen disease, muscles of the legs, hands, and/or eyelids are most affected, particularly those of the legs. Characteristic symptoms may include those mentioned above (e.g., difficulties starting to walk or run, arising from a bed or chair, relaxing the hand grip, etc.). In addition, some may experience spasms of other muscles, including certain muscles of the face, trunk, or other regions. For example, involvement of muscles that control movements of the eyeballs (extraocular muscles) may lead to temporary episodes of double vision or abnormal deviation of one eye in relation to the other (strabismus). In addition, in some cases, if myotonia affects muscles of the throat, affected individuals may have difficulties with chewing, swallowing, and/or attempting to talk after a long period of silence.

In individuals with Becker type myotonia congenita, symptoms tend to become apparent between the ages of four to 12 years. However, in some rare cases, onset may occur as late as approximately 18 years of age. Associated symptoms are similar to those of Thomsen disease; however, they tend to progress over many years and to be more severe.

In most cases of Becker disease, the myotonia is initially apparent in muscles of the legs. With disease progression, muscles of the arms, trunk, and face are eventually affected. Enhancement of symptoms by cold exposure appears to be less common than seen with Thomsen disease.

Abnormal muscle enlargement (hypertrophy) is typically present in individuals with Becker disease and may be particularly striking, resulting in an even more pronounced “body-builder like” appearance. In addition, in contrast to Thomsen type myotonia congenita, muscle weakness and muscle wasting (atrophy) may also be present.

According to the medical literature, two sisters with Becker type myotonia congenita demonstrated susceptibility to malignant hyperthermia (MH). MH is a potentially life-threatening condition that may be triggered due to exposure to certain general anesthetics (e.g., halothane) or skeletal muscle relaxants (e.g., succinylcholine) that may be administered during surgical procedures. Such a reaction may be characterized by a dangerous, sudden rise in body temperature (hyperthermia); stiffness of skeletal muscles; low blood pressure (hypotension); an irregular heartbeat (arrhythmias); and/or other complications, requiring immediate emergency intervention. In the two sisters described, exposure to succinylcholine resulted in generalized muscle rigidity without a rise in body temperature. According to researchers, it is unclear whether this was an incidental finding or whether MH or an MH-like condition may be associated with Becker disease in some cases. Therefore, the implications of this finding are not yet fully understood.

Causes

Thomsen type myotonia congenita is transmitted as an autosomal dominant trait. Genetic diseases are determined by two genes, one received from the father and one from the mother. Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary for the appearance of the disease. The abnormal gene can be inherited from either parent, or can be the result of a new mutation (gene change) in the affected individual. The risk of passing the abnormal gene from affected parent to offspring is 50% for each pregnancy regardless of the sex of the resulting child.

Becker type myotonia congenita is inherited as an autosomal recessive trait. Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%.

According to reports in the medical literature, parents of several individuals with Becker disease have been closely related by blood (consanguineous). With closely related parents, there may be an increased likelihood that both carry the same recessive disease gene, increasing the risk that their children may inherit the two genes necessary for the development of the disease.

Some cases have also been reported in which myotonia congenita has appeared to occur randomly for unknown reasons (sporadically) in the apparent absence of a family history.

Numerous mutations in the same gene have been identified in individuals with Thomsen and Becker types myotonia congenita. The gene, known as “CLCN1″, has been mapped to the long arm (q) of chromosome 7 (7q35). (For more on the CLCN1 gene, see below.) Chromosomes are found in the nucleus of all body cells. They carry the genetic characteristics of each individual. Pairs of human chromosomes are numbered from 1 through 22, with an unequal 23rd pair of X and Y chromosomes for males and two X chromosomes for females. Each chromosome has a short arm designated as “p” and a long arm identified by the letter “q”. Chromosomes are further subdivided into bands that are numbered. Therefore, “7q35″ refers to band 35 on the long arm of chromosome 7.

Thomsen and Becker types myotonia congenita are considered “ion channel diseases” or “channelopathies”, meaning that they result from abnormalities in the flow of certain ions across muscle cell membranes. Ions are particles that carry a positive or negative electrical charge. For example, positively charged ions (cations) include sodium and calcium; ions carrying a negative charge (anions) include chloride and phosphate.

As mentioned above, both Thomsen and Becker types myotonia congenita appear to be linked to “CLCN1″ and therefore are more specifically classified as “chloride channel diseases”. CLCN1 is a gene that is responsible for the normal functioning of certain chloride channels in voluntary (skeletal) muscle cell membranes. Ion channels, including chloride channels, are specialized proteins that regulate the flow or exchange of ions across certain cellular membranes. Chloride channels play an important role in regulating the skeletal muscle membranes’ electric response to stimuli (i.e., electric excitability). Researchers suspect that certain mutations of the CLCN1 gene result in reduced numbers or insufficient functioning of chloride channels (i.e., abnormally reduced chloride conductance), causing an impaired ability to maintain normal muscle excitability (electrical instability), associated myotonia, and other findings seen in myotonia congenita. Evidence also suggests that abnormal sodium channel activation may play some role in the hyperexcitability associated with the disease.

Affected Populations

Thomsen and Becker types myotonia congenita are thought to affect males and females in relatively equal numbers. Reported cases have included multiple affected members in multigenerational families (kindreds) as well as apparently sporadic cases. The reported incidence of these disorders is between .3 and .6 per 100,000 people in the general population.

Thomsen type myotonia congenita was initially described in 1876 by a Danish physician (Thomsen J). The disease affected the physician himself as well as multiple family members in several generations (approximately 64 family members in seven consecutive generations). The autosomal recessive form, designated Becker type myotonia congenita, was later described by another investigator (Becker PE).

According to reports in the medical literature, Thomsen disease tends to occur over multiple generations, usually without skipping generations. However, some rare exceptions have been noted in which skipped generations have been shown. As mentioned earlier, in individuals with Thomsen disease, the severity of associated symptoms may vary greatly among family members, with males typically more affected than females.

Becker type myotonia congenita has been reported in multiple siblings of unaffected parents. As noted above, in some of these cases, parents of children with Becker disease have been closely related by blood (consanguineous).

Related Disorders

Symptoms of the following disorders may be similar to those of myotonia congenita. Comparisons may be useful for a differential diagnosis:

Paramyotonia congenita is a rare genetic nonprogressive disorder that usually becomes apparent during infancy. The disorder is often characterized by myotonia triggered or aggravated by exposure to cold and alleviated by warm temperatures; in addition, the myotonia is sometimes described as “paradoxic,” since it tends to become more severe rather than to improve with exercise. Some with the disorder may also be affected by episodes of weakness or partial paralysis (paresis) that do not necessarily coincide with exposure to cold temperatures or myotonia. In addition, in some cases, periods of weakness may be induced by the administration of potassium. The disorder is not associated with muscle wasting or hypertrophy. Paramyotonia congenita is a sodium channel disease (sodium channelopathy). It results from mutations of a gene (called SCN4A) that has also been linked to hyperkalemic periodic paralysis (see below). The disease gene has been mapped to the long arm (q) of chromosome 17. Paramyotonia congenita is transmitted as an autosomal dominant trait. (For more information on this disorder, choose “paramyotonia congenita” as your search term in the Rare Disease Database.)

Hyperkalemic periodic paralysis is a rare genetic disorder that usually becomes apparent during infancy or childhood. The disorder is characterized by periodic episodes of muscle weakness that tend to occur during the daytime. In some cases, such episodes may also be characterized by myotonia. In addition, potassium levels may be abnormally increased in the blood (hyperkalemia) during episodes, potentially due to leakage of potassium from muscle. Reports suggest that such attacks may be triggered by exposure to cold temperatures (paramyotonia), rest, hunger, or administration of potassium. Hyperkalemic periodic paralysis is transmitted as an autosomal dominant trait and is due to mutations in the sodium channel gene SCN4A on chromosome 17. In addition, a number of additional neuromuscular disorders have been linked to defects of the SCN4A gene.

There are also a number of rare disease variants, including atypical painful myotonia, that have been described in the medical literature. Their possible relationship with Thomsen and Becker types myotonia congenita remains uncertain, pending further genetic studies. In addition, there are a number of other disorders (e.g., Schwartz Jampel syndrome, myotonic dystrophy) that may be characterized by myotonia, muscle stiffness and/or weakness, abnormal muscle enlargement (hypertrophy), and/or other symptoms similar to those that may occur in association with myotonia congenita. However, such disorders typically have other characteristic features that may differentiate them from Thomsen or Becker disease. (For more information on these disorders, choose the exact disease name in question as your search term in the Rare Disease Database.)

Diagnosis

Myotonia congenita may be diagnosed from infancy or early childhood to adulthood, based upon a thorough clinical evaluation, a detailed patient and family history, various specialized tests, and genetic analysis, if available.

In patients with myotonia congenita, clinical examination may reveal an inability to quickly release the hand grip, sustained muscle contraction after direct muscle percussion (percussion myotonia), and other characteristic findings. (During direct muscle percussion, the examining physician may lightly tap the contractile part of muscles with a small, hammerlike instrument [percussor].. In addition, specialized testing that records electrical activity in skeletal muscle at rest and during muscle contraction (electromyography [EMG]) typically demonstrates a repetitive discharge of electrical impulses (action potentials) after forceful contraction (myotonic discharges). According to reports in the medical literature, although people who carry one mutated gene for Becker disease typically do not have symptoms (asymptomatic heterozygous carriers), EMG testing may reveal myotonic discharges in some (particularly male) carriers (subclinical myotonia).

In some individuals, recommended diagnostic evaluation may include muscle biopsies. During a muscle biopsy, small samples of muscle tissue are removed and examined microscopically with the use of special stains (histochemistry). However, in those with myotonia congenita, such evaluation typically reveals only minimal abnormal changes (e.g., muscle fiber enlargement in hypertrophied muscle).

In some cases of suspected myotonia congenita, additional diagnostic studies may be recommended to help eliminate other diseases with similar symptoms and to confirm the diagnosis.

In rare cases, it is possible that laboratory studies in which skeletal muscle tissue is exposed to the anesthetic halothane (halothane contracture testing in vitro) may demonstrate findings consistent with a susceptibility to a malignant hyperthermia-like response.

Standard Therapies

Treatment

The treatment of Thomsen and Becker types myotonia congenita is directed toward the specific symptoms that are apparent in each individual. Such treatment may require the coordinated efforts of a team of medical professionals, such as pediatricians; specialists who diagnose and treat disorders of the skeleton, joints, muscles, and related tissues (orthopedists); physical therapists; and/or other health care professionals.

Specific therapies for the treatment of Thomsen and Becker diseases are symptomatic and supportive. In some cases, certain medications may be prescribed to help diminish muscle stiffness and other symptoms resulting from myotonia. (For more information, see the “Investigational Therapies” section below.) In addition, special exercises may be advised to help alleviate myotonic symptoms, since associated muscle rigidity may improve with proper movement and exercise of involved muscle groups.

As discussed earlier, two sisters with Becker type myotonia congenita demonstrated susceptibility to a malignant hyperthermia-like response. Although the implications of this finding are not fully understood, this potential risk must be taken into consideration by surgeons, anesthesiologists, dentists, and other health care workers when making decisions concerning surgery, the use of particular anesthetics, and the administration of certain medications.

Early intervention is important to ensure that affected children reach their potential. Special services that may be beneficial include special social support, physical therapy, and/or other medical, social, and/or vocational services.

Genetic counseling will be of benefit for affected individuals and their families. Other treatment for this disorder is symptomatic and supportive.

Investigational Therapies

Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222

TTY: (866) 411-1010

Email: prpl@cc.nih.gov

For information about clinical trials sponsored by private sources, contact:

www.centerwatch.com.

In some cases, as mentioned above, certain medications may be prescribed to individuals with Thomsen and Becker types myotonia congenita to diminish muscle stiffness and other symptoms resulting from myotonia. According to the medical literature, the most effective medications in such cases are those that help to stabilize the muscle membranes. Medications that have been prescribed to help treat myotonic symptoms have included anticonvulsant medications such as phenytoin, acetazolamide, and carbamazepine; skeletal muscle relaxants such as dantrolene sodium and quinine sulphate; the sodium channel blocking agent mexiletine; and/or, antihistamines such as trimeprazine. Additional studies are needed to determine the long-term safety and effectiveness of such medications in the treatment of myotonia associated with Thomsen and Becker types myotonia congenita.

According to the medical literature, one case of myotonia congenita showed dramatic improvement in response to therapy with carbamazepine. However continued research and clinical study are necessary to determine the long-term safety and effectiveness of carbamazepine as a potential treatment for individuals with myotonia congenita.

Resources

(Please note that some of these organizations may provide information concerning certain conditions potentially associated with this disorder [e.g., myotonia, risk for malignant hyperthermia or an MH-like condition, etc.].)

NORD's Rare Disease Database provides brief introductions for patients and their families to more than 1,200 rare diseases. This is not a comprehensive database since there are nearly 7,000 diseases considered rare in the U.S. We add new topics as we are able to do so, with the help of rare disease medical experts.

If you are seeking information about a rare disease that is not in this database, we would suggest contacting the Genetic and Rare Diseases Information Center (GARD) at the National Institutes of Health. NIH has the most complete database of rare diseases in the U.S.

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